1. Field of the Invention
This invention relates to a method for rendering insulating polymeric materials electrically conductive by impregnation of their surface with a conductive polymer. More particularly, this invention relates to a method of forming electrically conductive polymer blends within the surface layer of a non-porous host polymer.
2. Description of the Prior Art
Electrically conductive polymers are the focus of considerable interest as they are possible replacements for metallic conductors or semi-conductors in a wide variety of applications such as batteries, photovoltaics, electrostatic dissipation and electromagnetic shielding. Potential advantages of conductive polymers over conventional metallic conductors lie in their potential for light weight, lower cost and greater versatility in terms of synthesis and fabrication. However, most electrically conductive polymer systems produced to date are not sufficiently stable or processable for use in such application areas.
Conductive polymer blends (i.e., conductive polymers which are intimately mixed with one or more conventional polymers) have been prepared by several workers as a possible means of achieving improved environmental stability and/or mechanical properties for a given conductive polymer. Another potentially advantageous aspect of such polymer blends is that processing of the conductive polymer component becomes unnecessary if it can be synthesized within a suitably fabricated host polymer matrix, e.g., a thin film or fiber.
Particularly attractive for incorporation into conductive polymer blends would be the conductive polymers polypyrrole, polyaniline and their analogs. See Mol. Cryst. Liq. Cryst., 1982, Vol. 83, pp. 253-264, "Preparation and Characterization of Neutral and Oxidized Polypyrrole Films", G. B. Street, T. C. Clarke, M. Krounbi, K. Kanazawa, V. Lee, P. Pfluger, J. C. Scott and G. Weiser, and references therein (polypyrrole); see "Aqueous Chemistry and Electrochemistry of Polyacetylene and `Polyaniline`: Application to Rechargeable Batteries", pp. 248 and 249," A. J. Macdiarmid, J. C. Chiang, M. Halpern, W. S. Huang, J. R. Karwczyk, R. J. Mammone, S. L. Mu, N. L. D. Somasiri and W. Wu (polyaniline). These polymers exhibit good environmental stability and are potentially inexpensive to produce but suffer from a need for improved mechanical properties and processibility.
Incorporation of polypyrrole into polymer blends has been achieved by an electrochemical process wherein the pyrrole monomer is oxidatively polymerized to polypyrrole at a polymer-coated electrode. By this process, polypyrrole can be impregnated into the polymer which coats the electrode. See J. Chem. Soc., Chem. Commun., 1984, pp. 1015 and 1016, "Conductive Composites from Poly9vinylchloride) and Polypyrrole", M. A. DePaoli, R. J. Waltman, A. F. Diaz and J. Bargon and J. Chem. Soc., Chem. Commun., 1984, pp. 817 and 818, "Electrochemical Polymerization of Pyrrole and Polymer-coated Electrodes", O. Niwa and T. Tamamura. Two limitations of this process are that it is only applicable to uniform thin films of polymers and that it would be difficult to perform on a large scale due to electrode surface area limitations coupled with the apparent requirement of using a batch-type process.
In this respect, a general chemical process, rather than an electrochemical process, for the formation of polypyrrole blends could be operable on a much wider variety of materials, particularly thick films, fibers and molded polymeric articles. It is well known that pyrrole reacts with a number of chemical oxidizing agents, most notably trivalent iron compounds, to produce the so-called "pyrrole blacks", which are qualitatively identical in appearance, electrical properties and basic chemical composition, to the aforementioned polypyrrole. (4) See G. P. Gardini, "The Oxidation of Monocyclic Pyrroles", Adv. Heterocycl. Chem. 15, 67-98 (1973), and references therein, and Polymer Preprints, Japan, Vol. 33, p. 495 (1984) (reference attached).
Based on this chemical polypyrrole formation, absorption of oxidizing agents, particularly iron trichloride, into paper and other porous materials has been used as a method of activating such substances towards polypyrrole formation within the interstitial spaces when subsequently treated with pyrrole. See Journal of Electronic Materials, Vol. 13, No. 1, 1984, pp. 211-230, "Some Properties of Polypyrrole-Paper Composites", R. B. Bjorklund and I. Lundstroem, and German Application No. P 33 21 281.3, published Dec. 22, 1983. This process affords conductive composite materials. However, this process does not afford a general method of impregnating polypyrrole into non-porous polymeric materials to produce conductive polymer blends, nor is it likely such a process was contemplated, because the oxidant cannot be effectively absorbed into such non-porous polymeric materials unless it is soluble in the polymer itself.
With respect to the conductive polymer polyaniline, no prior art relating to the incorporation of polyaniline into polymer blends is known to Applicants at this time.